Depressions having a specifiable geometry are able to be applied in metallic workpieces by electrochemical metal working (EMC). In order to avoid that the depression widens radially because of the dissolving of the walls, the working gap between the electrode and the workpiece is selected to be as small as possible. Oscillating electrodes are usually used for this purpose. These, in each case, emit a current pulse when the distance between the electrode and the workpiece is at its smallest. The gap between the workpiece and the electrode is designated as working gap, in this instance.
At this time, evaluations of the voltage signal or the current signal are used to regulate the working gap. Methods are also known which make use of the measurement of a resistance. These methods each permit a relative measurement and determination of a working gap. In doing so, they are based on the fact that the measured signals vary as a function of the magnitude of the working gap. Thus, the resistance goes down with a decreasing working gap, for example.
The disadvantage of the methods in which the working gap arises from the evaluation of the voltage signal or the current signal, or from measurements of the resistance is that the measured signals depend on the state of the electrolyte in the working gap. Thus, the composition of the electrolyte changes, for instance, during the processing, that is, during the time a current or voltage pulse is applied to the electrodes. This change takes place, for example, by the electrolysis of the water or the solution of metal ions from the workpiece. The result is a superposition of the effects of the approach, for instance, the reduction in the measured resistance with falling gap size, with effects of the processing itself, such as an additional reduction in the resistance caused by an increase in the conductivity of the electrolyte by the absorption of dissolved metal ions.